The goal of these studies is to determine the role(s) of the integral membrane subunit ASGP-2 of the glycoprotein Muc4/SMC (sialomucin complex) as a ligand/modulator of the receptor tyrosine kinase ErbB2/HER2/Neu. Muc4 can activate specific phosphorylation of ErbB2 on Tyr1248 (implicated in neoplastic transformation) and potentiate neuregulin-induced phosphorylation of both ErbB2 and ErbB3. We have also demonstrated co-localization of and complex formation between Muc4 and ErbB2 in normal epithelial tissues. Our hypothesis is that Muc4 modulates ErbB2 signaling by activating specific pho-phorylation sites of the receptor and by regulating the localization on the receptor in polarized epithelial cells. We propose that complex formation between Muc4 and ErbB2 activates a survival pathway in epithelial cells and carcinomas. Tiffs survival mechanism affords both epithelial cells and tumor cells protection from apoptotic killing. To address the nature and roles of the Muc4-ErbB2 complex, we will use three approaches. 1) We will test the hypothesis that the Muc4-ErbB2 complex forms during transit to the cell surface. This early complex formation results in intracellular phosphorylation of the receptor and apical localization of the ASGP-2-bound ErbB2. 2) We will analyze the specific sites of phosphorylation of ErbB2 and ErbB3 mediated by ASGP-2/neuregulin activation. From these data we will predict and analyze the recruited pathway components of the ErbB2 and ErbB3 signaling complexes formed. These studies will constitute an initial step toward understanding the proximal steps of the signaling pathways initiated by, the Muc4-ErbB2 and Muc4-ErbB2-ErbB3-neuregulin signaling complexes, which trigger different celular responses. 3) We will investigate the role and mechanism for Muc4 as a survival factor/repressor of apoptosis. In particular, we will analyze the apoptosis pathway(s) (extrinsic and/or intrinsic) repressed by Muc4 and test the hypotheses that ErbB2 and the cell cycle inhibitor p27 kip are involved in this repression of apoptosis. In these studies we will use two methods for investigating ASGP-2 effects: upregulation of its expression by transduction or transfection in cells which do not normally, express Muc4 and downregulation of its expression by ribozyme (or antisense) methods in epithelial cells which do express Muc4. We will focus our attention primarily on polarized epithelial models to couple our model for ErbB2 localization with signal complex formation and cell functions. These studies will test a new paradigmjbr ErbB2 signaling as well as provide insights into molecular mechanisms for theASGP-2-mediated regulation of cell survival in epithelia and in tumors in which the Muc4 and ErbB2 are co-expressed.